1. Field of the Invention
The present invention relates to a method of preparing a reinforced polymer having improved mechanical properties.
2. Background Art
Plastics usage in the automotive industry is steadily increasing due to their light weight and continual improvements in mechanical properties. Currently, polymer-based materials may comprise at least percent of a given vehicle""s weight. These materials are used in various automotive components, e.g., interior and exterior trim and side panels. As pressures to improve fuel economy continue, more steel and aluminum parts may be targeted for replacement by polymer-based materials. Thus, improvements in the mechanical properties of polymers are necessary in order to meet more stringent performance requirements. Mechanical properties of a polymer include stiffness, dimensional stability, modulus, heat deflection temperature, barrier properties, and rust and dent resistance, to name a few. Improved mechanical properties may reduce manufacturing costs by reducing the part thickness and weight of the manufactured part and the manufacturing time thereof.
There are a number of ways to improve the mechanical properties of a polymer, including reinforcement with particulate fillers or glass fibers. Currently, it is known that polymers reinforced with nanometer-sized platelets or particles of layered silicates or clay can impact significant improvements in mechanical properties at much lower loading than conventional fillers. This type of composite is termed a xe2x80x9cnanocompositexe2x80x9d. More specifically, polymer-silicate nanocomposites are compositions in which nano-sized particles of a layered silicate, e.g., montmorillonite clay, are dispersed into a thermoplastic or a thermoset matrix. The improvement in mechanical properties of nanocomposites is due to a combination of factors, such as high aspect ratio and surface area of the particles.
Typically, two steps are involved in producing reinforced polymers or polymer nanocomposites. The first step involves a process of conditioning or preparing the clay to make it more compatible with a selected polymer. The conditioning step is performed because the clay is generally hydrophilic and many polymer resins of interest are hydrophobic, thus rendering the two relatively incompatible.
In its natural state, clay is made up of stacks of individual particles held together by ionic forces. A cation exchange may be performed to exchange a naturally occurring inorganic cation with an organic cation. In addition, this process may increase the interlayer spacing between each particle, lessening the attractive forces between them. This allows the clay to be compatible with the polymer for subsequent polymerization or compounding. This preparatory step is known as xe2x80x9ccation exchangexe2x80x9d. Generally, cation exchange is performed with a batch reactor containing an aqueous solution wherein an organic molecule, usually an alkyl ammonium salt, is dissolved into water along with the clay particles. The reactor is then heated. Once ion exchange takes place, the clay particles precipitate out and are then dried.
Depending on the polymer, a monomer may be further intercalated into the clay galleries. The organically modified clay is then ready for melt compounding to combine the clay with the polymer to make a workable material, or polymerizing monomers in the presence of the modified clay. Both the polymerization step and the melt compounding step involve known processing conditions in which the particles disperse and exfoliate in the polymer.
However, while this strategy is effective in more polar systems such as nylon, efforts to extend the technology to extremely non-polar polymers, such as polyolefins, have met with limited success. Many attempts at generating polyolefin based nanocomposites have been partially successful only by use of large amounts of compatibilizers. Not only is this prohibitively expensive, but the enhanced dispersion and exfoliation gained with the use of compatibilizers comes at the expense of matrix stiffness, since the compatibilizers are themselves of low molecular weight.
Additionally, because of the high melt viscosity of many thermoplastics, uniform dispersion of the layered silicate becomes difficult. On the other hand, in systems such as nylons and epoxies which can be polymerized in situ, intercalation of monomers and exfoliation of silicate layers is less of an issue. In situ polymerization of polyolefins, however, is plagued by sensitive and unstable catalysts.
Therefore, the use of supercritical fluids to both pretreat/delaminate silicate layers and to reduce melt viscosity is an attractive means of generating a well-exfoliated polymer silicate nanocomposite material for any material system.
Accordingly, it is an object of the present invention to provide an improved system and method of delaminating a layered silicate for producing reinforced polymers having improved mechanical properties.
The present invention also provides an improved method of preparing a reinforced polymer wherein the method includes providing particles or platelets of the layered silicate and a supercritical fluid and mixing the layered silicate particles with a polymer to form a treatable silicate-polymer mixture. The method further includes contacting the treatable mixture with the supercritical fluid and catastrophically or immediately depressurizing the contacted mixture to exfoliate the layered silicate particles so that the layered particles are substantially dispersed within the polymer to define the reinforced polymer.
The present invention also provides an improved reinforced polymer. The reinforced polymer includes a polymer mixed with a layered silicate having particle layers exfoliated by a supercritical fluid. The silicate layers are substantially exfoliated and dispersed within the polymer to provide reinforcement thereto.